Devibhagavatapurana 7.35.42

7.35.42

ānandasadanaṃ tattu puruṣādhiṣṭhitaṃ param |
tadūrdhvaṃ tu viśuddhākhyaṃ dalaṃ ṣoḍaśapaṅkajam ||

Above it is situated the Vis'uddha Chakra of the sixteen petals, comprising the sixteen letters a, â, i, î, u, û, ri, ri, li, lri, e, ai, o, ar, am, ah. This is of a smoky colour, highly lustrous, and is situated in the throat. The Jîvâtmâ sees the Paramâtmâ (the Highest Self) here and it is purified; hence it is called Vis'uddha. This wonderful lotus is termed Âkâs'a.